Field of the Invention
This invention relates to an interference wave removing apparatus, and more particularly to an interference wave removing apparatus which can remove, when a strong interference wave is present in a transhorizon microwave communication channel which employs angle diversity, a wide band interference wave and can adaptively equalize waveform distortion by fading.
Description of the Related Art
Microwave transhorizon communication or microwave communication beyond line-of-sight makes use of a scattering phenomenon of microwaves in the troposphere. First, microwave transhorizon communication will be described with reference to FIG. 1.
Transmission point A and reception point B are located on surface 401 of the earth. A microwave radiated from transmission point A propagates in the directions of vectors AC, AL and AU in accordance with the extension of directivity pattern of transmission antenna 403. In troposphere 402, the beams are scattered and partially received by reception antenna 404 at reception point B. In this instance, multi-path propagation along routes ALB, ACB and AUB as shown in FIG. 1 arises from a difference among locations at which the troposcatter takes place. While actually a multi-path wave arising from a very great number of routes is received, here it is modeled into three waves for simplified description. When such impulse signal 405 as shown in FIG. 1A is transmitted at transmission point A, impulse response 406 for the impulse signal at reception point B is delayed and diffused as shown in FIG. 1B. The signals of the routes undergo Rayleigh fading independently of each other, and impulse response 406 fluctuates in time and causes intense frequency selective fading.
In a radio channel which suffers from a high degree of multi-path fading, a diversity system or an adaptive equalization technique is essential, and in a channel having a long propagation distance such as a transhorizon communication, a matched filter (MF) and an equalizer of the decision feedback type (DFE) are developed as an important technique for canceling inter symbol interference.
An angle diversity method is one of diversity techniques which are used for transhorizon communication. Reception based on angle diversity is performed by preparing two reception horns for reception antenna 404 and setting the center axis of the reception pattern of one of the horns to the main beam (vector CB) direction while setting the center axis of the reception pattern of the other horn to the angle beam (vector UB) direction to construct two independent diversity branches. The reception signals from the main beam and the angle beam have no correlation to each other and can be used sufficiently as diversity signals. Further, since the propagation distance of route AUB is longer than that of route ACB, the angle beam reception wave is delayed by time .tau. corresponding to a route difference comparing with the main beam reception wave. It is to be noted that the propagation time of the main beam and the propagation time of the angle beam are fluctuated in time by the elevation angle of the reception horn for the angle beam or the propagation condition of the transmission waves.
By the way, in transhorizon communication, a comparatively high transmission power is required but the reception signal level is very low because the propagation distance is long and a scattering phenomenon in the troposphere is utilized. Therefore, an intense interference wave called near-end disturbing wave is liable to exist at the reception point. In some cases, power U of the interference wave becomes higher than power D of a desired wave from the other party, that is, D/U becomes a negative value lower than 0 dB. When such intense interference wave is present, interference by an FM channel, interference from an adjacent channel or an intentional jammer wave sometimes causes trouble with a digital microwave communication channel which employs PSK (phase shift keying) or QAM (quadrature amplitude modulation). In high speed digital transmission, an FM interference wave can be regarded as a narrow band interference wave, but any other interference wave may be a wide band wave.
Conventionally, in order to remove a intense wide band interference wave described above, a power-inversion adaptive array wherein interference waves are combined in the opposite phases to each other between the diversity branches is employed frequently. The technique is described, for example, in R. T. Compton Jr., "The Power-Inversion Adaptive Array: Concept and Performance". IEEE Transaction on Aerospace and Electronic Systems, Vol. AES-15, No. 6, pp. 803-814, November, 1979.
FIG. 2 shows a construction of a conventional reception system for transhorizon communication which employs the power-inversion adaptive array technique. Main beam horn 503 and angle beam horn 502 are provided on reception antenna 501, and receivers 504 and 505 each including a PSK demodulator are connected to horns 502 and 503, respectively. AGC amplifiers 506 and 507 for stabilizing the base band level are provided at the output sides of receivers 504 and 505, respectively.
Multiplier 510 and correlator 512 are provided on the output side of AGC amplifier 506. Multiplier 510 inputs the output of amplifier 506 and the output of correlator 512 and outputs the product of the input signals. Similarly, multiplier 511 and correlator 513 are provided on the output side of the other AGC amplifier 507. Multiplier 511 inputs the output of amplifier 507 and the output of correlator 513 and outputs the product of the input signals.
Adder 515 for calculating the sum of the output of multiplier 510 and the output of multiplier 511 and subtractor 516 for calculating the difference between the output of multiplier 510 and the output of multiplier 511 are provided. AGC amplifier 508 is provided on the output side of adder 515, and AGC amplifier 509 is provided on the output side of subtractor 516. Switching unit 517 for selecting one of the outputs of AGC amplifiers 508 and 509 is provided, and switching unit 517 is controlled by switch controller 522. The output of AGC amplifier 508 is fed back to correlators 512 and 513. Adaptive matched filter (AMF) 518 and equalizer 519 of the decision feedback type are connected in series to the output side of switching unit 517, and the output of decision feedback equalizer 519 is supplied as decision data to the outside. The decision data is supplied also to switch controller 522.
Next, operation of the system will be described. Inputs to horns 502 and 503, that is, diversity inputs, are demodulated by receivers 504 and 505, respectively, and then, level variation components by fading are removed from the diversity inputs by AGC amplifiers 506 and 507, respectively, whereafter the diversity inputs are supplied to multipliers 510 and 511, respectively. Complex tap coefficients are supplied from correlators 512 and 513 to multipliers 510 and 511, respectively, and the input signals are multiplied by the complex tap coefficients by multipliers 510 and 511, respectively. Each tap coefficient is correlation value of each of the outputs of AGC amplifiers 506 and 507 to the output of AGC amplifier 508 after diversity combining. The correlation values are complex conjugate with transfer coefficients of the input signals of multipliers 510 and 511. Consequently, the outputs of multipliers 510 and 511 are same in phase with each other. Further, each of the outputs of multipliers 510 and 511 has an amplitude equal to a square of an input value to it. Accordingly, maximum ratio combining is achieved by combining the output of multiplier 510 with the output of multiplier 511 by means of adder 515.
When no interference wave exists, switching unit 517 is controlled by switch controller 522 to select and output the output of adder 515, that is, the maximum ratio combiner routes of the outputs of AGC amplifiers 508 and 509. When a channel watchman watches the symbol error rate or the signal-to-noise ratio representative of the channel quality and finds out significant deterioration of the symbol error rate caused by intense interference, switch controller 522 will be manually operated to control switching unit 517 to change over the output thereof so as to select subtractor 516 side. Subtractor 516 subtracts the output of multiplier 511 from the output of multiplier 510 to perform opposite phase combining to remove an interference wave. In other words, the output of subtractor 516 is equivalent to the power-inversion adaptive array output.
Next, the principle of removal of an interference wave in the present system will be described with reference to a vector combining diagram of FIG. 3. The input to angle beam horn 502 side is represented by input I, and the input to main beam horn 503 side is represented by input II. Further, desired waves are represented by S1 and S2, and interfere waves are represented by J1 and J2. When input I shown in (a) is inputted to angle beam horn 502 and input II shown in (b) is inputted to main beam horn 503, the outputs of multipliers 510 and 511 are shown in (b) and (e) and the outputs of adder 515 and subtractor 516 are shown in (c) and (f), respectively.
When the interference waves are higher than the desired waves, that is, when the interference waves are so high that D/U is lower than 0 dB, the level of the interference waves is higher than the level of the desired waves at the output of adder 515, and the interference waves are fed back as reference signals to correlators 512 and 513 to combine the interference waves with each other in the same phase relation. In particular, at each of multipliers 510 and 511, the input signal is multiplied by such a weight coefficient as combine the interference waves between the diversity branches at a maximum rate. As a result, as seen from (b) and (e), the interference waves J1 and J2 become equal in amplitude and phase at the outputs of multipliers 510 and 511. In this instance, at adder 515, same phase combination of the interference waves proceeds as seen from (c).
Meanwhile, subtractor 516 performs an operation opposite to maximum ratio combining at adder 515. Since the levels of the interference waves at the inputs of subtractor 516 are individually normalized by AGC amplifiers 506 and 507, opposite phase combining of the interference waves is performed as seen from (f) at subtractor 516 to remove the interference waves. As a result, subtractor 516 extracts only the desired signal waves. However, as regards desired waves S1 and S2, neither maximum ratio combining nor same phase combining is performed. The desired signal waves may be canceled depending upon the relationship in phase between the desired waves S and the interference waves J.
Here, it is assumed that input I and input II have such a relationship in amplitude and phase as shown in (g) and (j) in FIG. 3 wherein the differences of the phases of the interference waves from the desired waves are equal to each other and the amplitudes of them are equal to each other. The outputs of multipliers 510 and 511 then are shown in (h) and (k), respectively, and the output of adder 515 is shown in (i) while the output of subtractor 516 is shown in (l). As seen from those figures, the outputs of multipliers 510 and 511 coincide completely with each other. As a result, adder 515 performs opposite phase combining for both of interference waves I and desired waves S, and adder 516 performs opposite phase combining for both of interference waves I and desired waves S. In other words, while the interference waves are removed, also the desired signal waves disappear simultaneously.
As described above, with the conventional interference wave removing apparatus based on the power-inversion adaptive array system, when it is tried to remove interference waves, since maximum ratio combining or same phase combining of desired waves between the diversity branches is not performed, optimum reception by adaptive equalization and removal of the interference waves in a channel having multi-path fading are not consistent with each other, and in the worst case, the desired signals are canceled.
As an interference wave removing technique other than the power-inversion adaptive array method, a method wherein the null of the antenna pattern, that is, a deep dip in the directivity characteristic, is provided in the direction of an interference wave so as not to receive the interference wave is conventionally employed. The method is sometimes called side lobe cancellor. With this method, when the coming directions of a desired wave and an interference wave coincide with each other, also the desired wave cannot be received.